Pressure-sensitive adhesives (PSA) for duct tape applications are normally based on natural rubber and/or synthetic elastomers such as polyisoprene. Because untreated natural rubber has a molecular weight greater than about 400,000, it needs to be broken down to more useful molecular weights (e.g., about 100,000) for use in pressure-sensitive adhesives.
The high molecular weight of natural rubber is usually reduced by mastication in a batch process using a Banbury or similar mixer. A typical formulation includes natural rubber, filler, tackifier, and optionally a process oil. “Constant viscosity” grades of rubber, such as “CV60,” which has a Mooney viscosity of 60, are available. However, because of the extra processing steps needed, these CV grades are relatively expensive. “Technically specified rubber” (or “TSR” grade rubber) has higher molecular weight and is less expensive. TSR grades of rubber are produced in a range of Mooney viscosities depending on the plantation and country of origin. Processing time required in a Banbury mixer depends on the initial molecular weight of the rubber. In general, higher molecular weight rubbers require a longer mixing time, which hampers productivity.
Extrusion processes for making pressure-sensitive adhesives are known. U.S. Pat. Nos. 5,539,033 and 6,166,110 disclose a continuous extrusion process for making a PSA. Single sources or blends of rubbers are taught as suitable. The references teach to use natural rubbers such as CV60, ribbed smoked sheet, and synthetic rubbers such as polyisoprene and styrene-butadiene rubber (SBR). U.S. Pat. No. 6,777,490 teaches to use an extruder to process an aqueous rubber latex mixture. Relatively high temperatures (up to 170° C.) are needed to evaporate water from the formulation during adhesive processing. U.S. Pat. Nos. 6,506,447; 6,780,271; and 7,476,416 teach to use a modified planetary roller extruder to process rubber. The '447 and '271 patents refer to the process as “mastication-free,” while the '416 patent refers to “purposeful mastication” in the planetary roller extruder. According to the '447 and '271 patents, suitable planetary roller extruders can have 7-24 screws or spindles. In general, planetary roller extruders are far more expensive to obtain and maintain when compared with single-screw or twin-screw extruders.
Although low capital investment, high production rates, and low processing costs have made the continuous extrusion process for making PSAs more attractive in recent years, there are several important drawbacks.
We found, for instance, that continuous extrusion processes can provide homogeneous adhesives at high throughput rates when the natural rubber has low to medium molecular weight (as with the CV60 natural rubber described in the '033 and '110 patents). However, when higher molecular weight natural rubbers are used, longer residence times are needed in the extruder to achieve adequate mastication of the rubber. Consequently, the high throughput rates that are a principal advantage of continuous extrusion are sacrificed.
Another option is to add a peptizer (e.g., zinc soaps of unsaturated fatty acids or aromatic disulfides) into the rubber formulation to chemically assist in the breakdown of the high-molecular-weight rubber in the extruder without reducing throughput. However, residual peptizer in the PSA can disrupt aging properties of the resulting tape.
Natural rubber contains traces of various fatty acids, including stearic acid, oleic acid, linoleic acid, and others (see A. Arnold et al., “Role of Fatty Acids in Autooxidation of Deproteinized Natural Rubber, J. Nat. Rubber Res. 6 (1991) 75). Fatty acids are sometimes included in rubber compounding processes to enhance vulcanization, particularly in processing rubber for tire manufacture (see, e.g., GB 1,532,294 and U.S. Pat. No. 3,900,999), or to accelerate crosslinking by other active components such as zinc oxide (see, e.g., U.S. Pat. No. 6,780,271).
Duct tapes are constructed using a flexible backing layer, e.g., low-density polyethylene or the like, a cloth scrim, and a rubber adhesive that can penetrate openings in the scrim and bond the scrim to the backing layer (see, e.g., U.S. Pat. Nos. 4,303,724; 4,992,331; 5,108,815; 5,271,999). A challenge with duct tapes is in providing a high level of adhesion while also permitting clean removability of the tape after use. Duct tapes tend to leave behind considerable adhesive residue, particularly when the substrate to which the tape has been attached is marble, ceramic tile, laminate flooring, or carpeting.
A need remains for new ways to manufacture pressure-sensitive adhesives for tape applications from high-molecular-weight natural rubber in a continuous extrusion process without sacrificing productivity and without reliance on chemical peptizers or high process temperatures. Ideally, the expensive and complex planetary roller extruders could be avoided. The industry needs tapes that have strong adhesion on substrate surfaces when applied but leave no residue when the tape is subsequently removed, especially from difficult substrates.